Group III nitride based light emitting diode structures with a quantum well and superlattice, group III nitride based quantum well structures and group III nitride based superlattice structures

ABSTRACT

A light emitting diode is provided having a Group III nitride based superlattice and a Group III nitride based active region on the superlattice. The active region has at least one quantum well structure. The quantum well structure includes a first Group III nitride based barrier layer, a Group III nitride based quantum well layer on the first barrier layer and a second Group III nitride based barrier layer. A Group III nitride based semiconductor device and methods of fabricating a Group III nitride based semiconductor device having an active region comprising at least one quantum well structure are provided. The quantum well structure includes a well support layer comprising a Group III nitride, a quantum well layer comprising a Group III nitride on the well support layer and a cap layer comprising a Group III nitride on the quantum well layer. A Group III nitride based semiconductor device is also provided that includes a gallium nitride based superlattice having at least two periods of alternating layers of In X Ga 1−X N and In Y Ga 1−Y N, where 0≦X&lt;1 and 0≦Y&lt;1 and X is not equal to Y. The semiconductor device may be a light emitting diode with a Group III nitride based active region. The active region may be a multiple quantum well active region.

CROSS-REFERENCE TO PROVISIONAL APPLICATION

This application claims the benefit of, and priority from, ProvisionalApplication Ser. No. 60/294,445, filed May 30, 2001 entitledMULTI-QUANTUM WELL LIGHT EMITTING DIODE STRUCTURE, ProvisionalApplication Ser. No. 60/294,308, filed May 30, 2001 entitled LIGHTEMITTING DIODE STRUCTURE WITH SUPERLATTICE STRUCTURE and ProvisionalApplication Ser. No. 60/294,378, filed May 30, 2001 entitled LIGHTEMITTING DIODE STRUCTURE WITHMULTI-QUANTUM WELL AND SUPERLATTIC.ESTRUCTURE, the disclosures of which are hereby incorporated herein byreference in their entirety as if set forth fully herein.

FIELD OF THE INVENTION

This invention relates to microelectronic devices and fabricationmethods therefor, and more particularly to structures which may beutilized in Group III nitride semiconductor devices, such as lightemitting diodes (LEDs).

BACKGROUND OF THE INVENTION

Light emitting diodes are widely used in consumer and commercialapplications. As is well known to those having skill in the art, a lightemitting diode generally includes a diode region on a microelectronicsubstrate. The microelectronic substrate may comprise, for example,gallium arsenide, gallium phosphide, alloys thereof, silicon carbideand/or sapphire. Continued developments in LEDs have resulted in highlyefficient and mechanically robust light sources that can cover thevisible spectrum and beyond. These attributes, coupled with thepotentially long service life of solid state devices, may enable avariety of new display applications, and may place LEDs in a position tocompete with the well entrenched incandescent lamp.

One difficulty in fabricating Group III nitride based LEDs, such asgallium nitride based LEDs, has been with the fabrication of highquality gallium nitride. Typically, gallium nitride LEDs have beenfabricated on sapphire or silicon carbide substrates. Such substratesmay result in mismatches between the crystal lattice of the substrateand the gallium nitride. Various techniques have been employed toovercome potential problems with the growth of gallium nitride onsapphire and/or silicon carbide. For example, aluminum nitride (AlN) maybe utilized as a buffer between a silicon carbide substrate and a GroupIII active layer, particularly a gallium nitride active layer.Typically, however, aluminum nitride is insulating rather thanconductive. Thus, structures with aluminum nitride buffer layerstypically require shorting contacts that bypass the aluminum nitridebuffer to electrically link the conductive silicon carbide substrate tothe Group III nitride active layer.

Alternatively, conductive buffer layer materials such as gallium nitride(GaN), aluminum gallium nitride (AlGaN), or combinations of galliumnitride and aluminum gallium nitride may allow for elimination of theshorting contacts typically utilized with AlN buffer layers. Typically,eliminating the shorting contact reduces the epitaxial layer thickness,decreases the number of fabrication steps required to produce devices,reduces the overall chip size, and/or increases the device efficiency.Accordingly, Group III nitride devices may be produced at lower costwith a higher performance. Nevertheless, although these conductivebuffer materials offer these advantages, their crystal lattice matchwith silicon carbide is less satisfactory than is that of aluminumnitride.

The above described difficulties in providing high quality galliumnitride may result in reduced efficiency the device. Attempts to improvethe output of Group III nitride based devices have included differingconfigurations of the active regions of the devices. Such attempts have,for example, included the use of single and/or double heterostructureactive regions. Similarly, quantum well devices with one or more GroupIII nitride quantum wells have also been described. While such attemptshave improved the efficiency of Group III based devices, furtherimprovements may still be achieved.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a light emitting diodehaving a Group III nitride based superlattice and a Group III nitridebased active region on the superlattice. The active region has at leastone quantum well structure. The quantum well structure includes a firstGroup III nitride based barrier layer, a Group III nitride based quantumwell layer on the first barrier layer and a second Group III nitridebased barrier layer on the quantum well layer.

In further embodiments of the present invention, the light emittingdiode includes from about 2 to about 10 repetitions of the at least onequantum well structure.

In additional embodiments of the present invention, the superlatticeincludes a gallium nitride based superlattice having at least twoperiods of alternating layers of In_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N,where 0≦X<1 and 0≦Y<1 and X is not equal to Y. The first Group IIInitride based barrier layer provides a well support layer comprising aGroup III nitride and the second Group III nitride based barrier layerprovides a cap layer comprising a Group III nitride on the quantum welllayer.

In such embodiments, the cap layer may have a lower crystal quality thanthe well support layer.

In still further embodiments of the present invention, the well supportlayer comprises a gallium nitride based layer, the quantum well layercomprises an indium gallium nitride layer and the barrier layercomprises a gallium nitride based layer. In such embodiments, the wellsupport layer and the cap layer may be provided by layers ofIn_(X)Ga_(1−X)N where 0≦X<1. Furthermore, the indium composition of thewell support layer and the cap layer may be less than the indiumcomposition of the quantum well layer.

The well support layer and the cap layer may also be provided by a layerof Al_(X)In_(Y)Ga_(1−X−Y)N where 0<X<1, 0≦Y<1 and X+Y≦1. Furthermore,the well support layer and the cap layer may be undoped. Alternatively,the well support layer and the cap layer may have an n-type doping levelof less than about 5×10¹⁹ cm⁻³. The cap layer and the well support layermay also have a higher bandgap than the quantum well layer. The combinedthickness of the well support layer and the cap layer may be from about50 to about 400 Å. The thickness of the well support layer may begreater than a thickness of the cap layer. The quantum well layer mayhave a thickness of from about 10 to about 50 Å. For example, thequantum well layer may have a thickness of about 20 Å. Furthermore, thepercentage of indium in the quantum well layer may be from about 15% toabout 40%.

In additional embodiments of the present invention, a Group III nitridebased spacer layer is provided between the well support layer and thesuperlattice. The spacer layer may be undoped GaN.

In other embodiments of the present invention, the bandgap of thequantum well is less than the bandgap of the superlattice.

In further embodiments of the present invention, the light emittingdiode includes a second well support layer comprising a Group IIInitride on the cap layer, a second quantum well layer comprising a GroupIII nitride on the second well support layer and a second cap layercomprising a Group III nitride on the second quantum well layer.

In additional embodiments of the present invention, the gallium nitridebased superlattice comprises from about 5 to about 50 periods. Thealternating layers of In_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N may have acombined thickness of from about 10 to about 140 Å.

In particular embodiments of the present invention, X=0 for layers ofIn_(X)Ga_(1−X)N of the superlattice. In such embodiments, the InGaNlayers may have a thickness of from about 5 to about 40 Å and the GaNlayers may have a thickness of from about 5 to about 100 Å.

In further embodiments of the present invention, the gallium nitridebased superlattice is doped with an n-type impurity to a level of fromabout 1×10¹⁷ cm⁻³ to about 5×10¹⁹ cm⁻³. The doping level of the galliumnitride based superlattice may be an actual doping level of layers ofthe alternating layers. The doping level may also be an average dopinglevel of layers of the alternating layers. Thus, for example, the lightemitting diode may include doped Group III nitride layers adjacent thesuperlattice where the doped Group III nitride layers are doped with ann-type impurity to provide an average doping of the doped Group IIInitride layers and the superlattice of from about 1×10¹⁷ cm⁻³ to about5×10¹⁹ cm⁻³. The bandgap of the superlattice may be from about 2.95 eVto about 3.35 eV and, in certain embodiments, may be about 3.15 eV.

In other embodiments of the present invention, a Group III nitride basedsemiconductor device having an active region comprising at least onequantum well structure is provided. The quantum well structure includesa well support layer comprising a Group III nitride, a quantum welllayer comprising a Group III nitride on the well support layer and a caplayer comprising a Group III nitride on the quantum well layer.

The cap layer may have a lower crystal quality than the well supportlayer. The well support layer may be provided by a gallium nitride basedlayer, the quantum well layer may be provided by an indium galliumnitride layer and the barrier layer may be provided by a gallium nitridebased layer. In such embodiments, the well support layer and the caplayer may be provided by layers of In_(X)Ga_(1−X)N where 0≦X<1.Furthermore, the indium composition of the well support layer and thecap layer may be less the indium composition of the quantum well layer.Similarly, the well support layer and the cap layer may be provided bylayers of Al_(X)In_(Y)Ga_(1−X−Y)N where 0<X<1, 0≦Y<1 and X+Y≦1.

Furthermore, the well support layer and the cap layer may be undoped.Alternatively, the well support layer and the cap layer may have adoping level of less than about 5×10¹⁹ cm⁻³.

In further embodiments of the present invention, the cap layer and thewell support layer have a higher bandgap than the quantum well layer.The combined thickness of the well support layer and the cap layer maybe from about 50 to about 400 Å. For example, the combined thickness ofthe well support layer and the cap layer may be greater than about 90 Å.Similarly, the combined thickness of the well support layer and the caplayer may be about 225 Å. The thickness of the well support layer may begreater than the thickness of the cap layer.

In additional embodiments of the present invention, the quantum welllayer has a thickness of from about 10 to about 50 Å. For example, thequantum well layer may have a thickness of about 25 Å. Furthermore, thepercentage of indium in the quantum well layer may from about 5% toabout 50% .

In further embodiments of the Group III nitride based semiconductordevice according to the present invention, a superlattice is providedand the well support layer is on the superlattice. The superlattice mayhave a bandgap of about 3.15 eV. Furthermore, a Group III nitride basedspacer layer may be provided between the well support layer and thesuperlattice. The spacer layer may be undoped GaN. Also, the bandgap ofthe at least one quantum well may be less than the bandgap of thesuperlattice.

In still further embodiments of the present invention, a second wellsupport layer comprising a Group III nitride is provided on the caplayer. A second quantum well layer comprising a Group III nitride isprovided on the second well support layer; and a second cap layercomprising a Group III nitride is provided on the second quantum welllayer.

In particular embodiments of the present invention, the Group IIInitride based semiconductor device includes from about 2 to about 10repetitions of the at least one quantum well structures.

Embodiments of the present invention further provide a Group III nitridebased semiconductor device that includes a gallium nitride basedsuperlattice having at least two periods of alternating layers ofIn_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N, where 0≦X<1 and 0≦Y<1 and X is notequal to Y.

In further embodiments of the present invention, the gallium nitridebased superlattice includes from about 5 to about 50 periods. Forexample, the gallium nitride based superlattice may include 25 periods.Similarly, the gallium nitride based superlattice may include 10periods.

In additional embodiments of the present invention, the gallium nitridebased superlattice comprises from about 5 to about 50 periods. Thealternating layers of In_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N may have acombined thickness of from about 10 to about 140 Å.

In particular embodiments of the present invention, X=0 for layers ofIn_(X)Ga_(1−X)N of the superlattice. In such embodiments, the InGaNlayers may have a thickness of from about 5 to about 40 Å and the GaNlayers may have a thickness of from about 5 to about 100 Å.In stillfurther embodiments of the present invention, the gallium nitride basedsuperlattice is doped with an n-type impurity to a level of from about1×10¹⁷ cm⁻³ to about 5×10¹⁹ cm⁻³. The doping level of the galliumnitride based superlattice may be an actual doping level of layers ofthe alternating layers or may be an average doping level of layers ofthe alternating layers.

In certain embodiments of the present invention, doped Group III nitridelayers are provided adjacent the superlattice. The doped Group IIInitride layers are doped with an n-type impurity to provide an averagedoping of the doped Group III nitride layers and the superlattice offrom about 1×10¹⁷ cm⁻³ to about 5×10¹⁹ cm⁻³.

In additional embodiments of the present invention, a bandgap of thesuperlattice is about 3.15 eV.

In embodiments of the present invention where the Group III nitridebased semiconductor device comprises a light emitting diode, the lightemitting diode includes a Group III nitride based active region on thesuperlattice. Additionally, a Group III nitride based spacer layer mayalso be provided between the active region and the superlattice. Such aspacer layer may be undoped GaN.

In certain embodiments of the present invention, the active regioncomprises at least one quantum well. In such embodiments, a bandgap ofthe quantum well may be less than a bandgap of the superlattice.

Additional embodiments of the present invention provide a method offabricating a Group III nitride based semiconductor device having anactive region comprising at least one quantum well structure. Thequantum well structure is fabricated by forming a well support layercomprising a Group III nitride, forming a quantum well layer comprisinga Group III nitride on the quantum well support layer and forming a caplayer comprising a Group III nitride on the quantum well layer.

In particular embodiments of the present invention, forming a wellsupport layer comprising a Group III nitride is provided by forming thewell support layer at a first temperature. Forming a quantum well layeris provided by forming the quantum well layer at a second temperaturewhich is less than the first temperature. Forming a cap layer isprovided by forming the cap layer at a third temperature which is lessthan the first temperature. In certain embodiments of the presentinvention, the third temperature is substantially the same as the secondtemperature.

In further embodiments of the present invention, the well support layercomprises a gallium nitride based layer, the quantum well layercomprises an indium gallium nitride layer and the cap layer comprises agallium nitride based layer. In such embodiments, the first temperaturemay be from about 700 to about 900° C. Furthermore, the secondtemperature may be from about 0 to about 200° C. less than the firsttemperature. The indium gallium nitride layer may be formed in anitrogen atmosphere or other atmosphere.

Preferably, forming a well support layer and forming a cap layer areprovided by forming a cap layer of In_(X)Ga_(1−X)N, where 0≦X<1 andforming a well support layer of In_(X)Ga_(1−X)N, where 0≦X<1. Also, theindium composition of the well support layer and the cap layer may beless an indium composition of the quantum well layer.

In additional embodiments of the present invention, forming a wellsupport layer and forming a cap layer are provided by forming a caplayer of Al_(X)In_(Y)Ga_(1−X−Y)N, where 0<X<1, 0≦Y<1 and X+Y≦1 andforming a well support layer of Al_(X)In_(Y)Ga_(1−X−Y)N, where 0<X<1,0≦Y<1 and X+Y≦1.

Further embodiments of the present invention include forming asuperlattice, where the well support layer is on the superlattice.Additional embodiments of the present invention include, forming a GroupIII nitride based spacer layer between the well support layer and thesuperlattice. The spacer layer may be undoped GaN. Additionalembodiments of the present invention include forming a second wellsupport layer comprising a Group III nitride on the cap layer, forming asecond quantum well layer comprising a Group III nitride on the secondwell support layer and forming a second cap layer comprising a Group IIInitride on the second quantum well layer. In such embodiments, thesecond well support layer may be formed at substantially the firsttemperature, the second quantum well layer may be formed atsubstantially the second temperature which is less than the firsttemperature and the second cap layer formed at substantially the thirdtemperature which is less than the first temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be more readily understoodfrom the following detailed description of specific embodiments thereofwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a Group III nitride light emittingdiode incorporating embodiments of the present invention;

FIG. 2 is a schematic illustration of a Group III nitride light emittingdiode incorporating further embodiments of the present invention; and

FIG. 3 is a schematic illustration of a quantum well structure and amulti-quantum well structure according to additional embodiments of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout. It will beunderstood that when an element such as a layer, region or substrate isreferred to as being “on” or extending “onto” another element, it can bedirectly on or extend directly onto the other element or interveningelements may also be present. In contrast, when an element is referredto as being “directly on” or extending “directly onto” another element,there are no intervening elements present. Moreover, each embodimentdescribed and illustrated herein includes its complementary conductivitytype embodiment as well.

Embodiments of the present invention will be described with reference toFIG. 1 that illustrates a light emitting diode (LED) structure 40. TheLED structure 40 of FIG. 1 includes a substrate 10, which is preferably4H or 6H n-type silicon carbide. Substrate 10 may also comprisesapphire, bulk gallium nitride or another suitable substrate. Alsoincluded in the LED structure 40 of FIG. 1 is a layered semiconductorstructure comprising gallium nitride-based semiconductor layers onsubstrate 10. Namely, the LED structure 40 illustrated includes thefollowing layers: a conductive buffer layer 11, a first silicon-dopedGaN layer 12, a second silicon doped GaN layer 14, a superlatticestructure 16 comprising alternating layers of silicon-doped GaN and/orInGaN, an active region 18, which may be provided by a multi-quantumwell structure, an undoped GaN and/or AlGaN layer 22, an AlGaN layer 30doped with a p-type impurity, and a GaN contact layer 32, also dopedwith a p-type impurity. The structure further includes an n-type ohmiccontact 23 on the substrate 10 and a p-type ohmic contact 24 on thecontact layer 32.

Buffer layer 11 is preferably n-type AlGaN. Examples of buffer layersbetween silicon carbide and group III-nitride materials are provided inU.S. Pat. Nos. 5,393,993 and 5,523,589, and U.S. application Ser. No.09/154,363 entitled “Vertical Geometry InGaN Light Emitting Diode”assigned to the assignee of the present invention, the disclosures ofwhich are incorporated by reference as if fully set forth herein.Similarly, embodiments of the present invention may also includestructures such as those described in U.S. Pat. No. 6,201,262 entitled“Group III Nitride Photonic Devices on Silicon Carbide Substrates WithConductive Buffer Interlay Structure,” the disclosure of which isincorporated herein by reference as if set forth fully herein.

GaN layer 12 is preferably between about 500 and 4000 nm thick inclusiveand is most preferably about 1500 nm thick. GaN layer 12 may be dopedwith silicon at a level of about 5×10¹⁷ to 5×10¹⁸ cm⁻³. GaN layer 14 ispreferably between about 10 and 500 Å thick inclusive, and is mostpreferably about 80 Å thick. GaN layer 14 may be doped with silicon at alevel of less than about 5×10¹⁹ cm⁻³.

As illustrated in FIG. 1, a superlattice structure 16 according toembodiments of the present invention includes alternating layers ofIn_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N, wherein X is between 0 and 1inclusive and X is not equal to Y. Preferably, X=0 and the thickness ofeach of the alternating layers of InGaN is about 5–40 Å thick inclusive,and the thickness of each of the alternating layers of GaN is about5–100 Å thick inclusive. In certain embodiments, the GaN layers areabout 30 Å thick and the InGaN layers are about 15 Å thick. Thesuperlattice structure 16 may include from about 5 to about 50 periods(where one period equals one repetition each of the In_(X)Ga_(1−X)N andIn_(Y)Ga_(1−Y)N layers that comprise the superlattice). In oneembodiment, the superlattice structure 16 comprises 25 periods. Inanother embodiment, the superlattice structure 16 comprises 10 periods.The number of periods, however, may be decreased by, for example,increasing the thickness of the respective layers. Thus, for example,doubling the thickness of the layers may be utilized with half thenumber of periods. Alternatively, the number and thickness of theperiods may be independent of one another.

Preferably, the superlattice 16 is doped with an n-type impurity such assilicon at a level of from about 1×10¹⁷ cm⁻³ to about 5×10¹⁹ cm⁻³. Sucha doping level may be actual doping or average doping of the layers ofthe superlattice 16. If such doping level is an average doping level,then it may be beneficial to provide doped layers adjacent thesuperlattice structure 16 that provide the desired average doping whichthe doping of the adjacent layers is averaged over the adjacent layersand the superlattice structure 16. By providing the superlattice 16between substrate 10 and active region 18, a better surface may beprovided on which to grow InGaN-based active region 18. While notwishing to be bound by any theory of operation, the inventors believethat strain effects in the superlattice structure 16 provide a growthsurface that is conducive to the growth of a high-qualityInGaN-containing active region. Further, the superlattice is known toinfluence the operating voltage of the device. Appropriate choice ofsuperlattice thickness and composition parameters can reduce operatingvoltage and increase optical efficiency.

The superlattice structure 16 may be grown in an atmosphere of nitrogenor other gas, which enables growth of higher-quality InGaN layers in thestructure. By growing a silicon-doped InGaN/GaN superlattice on asilicon-doped GaN layer in a nitrogen atmosphere, a structure havingimproved crystallinity and conductivity with optimized strain may berealized.

In certain embodiments of the present invention, the active region 18may comprise a single or multi-quantum well structure as well as singleor double heterojunction active regions. In particular embodiments ofthe present invention, the active region 18 comprises a multi-quantumwell structure that includes multiple InGaN quantum well layersseparated by barrier layers (not shown in FIG. 1).

Layer 22 is provided on active region 18 and is preferably undoped GaNor AlGaN between about 0 and 120 Å thick inclusive. As used herein,undoped refers to a not intentionally doped. Layer 22 is preferablyabout 35 Å thick. If layer 22 comprises AlGaN, the aluminum percentagein such layer is preferably about 10–30% and most preferably about 24%.The level of aluminum in layer 22 may also be graded in a stepwise orcontinuously decreasing fashion. Layer 22 may be grown at a highertemperature than the growth temperatures in quantum well region 25 inorder to improve the crystal quality of layer 22. Additional layers ofundoped GaN or AlGaN may be included in the vicinity of layer 22. Forexample, LED 1 may include an additional layer of undoped AlGaN about6–9 Å thick between the active region 18 and the layer 22.

An AlGaN layer 30 doped with a p-type impurity such as magnesium isprovided on layer 22. The AlGaN layer 30 may be between about 0 and 300Å thick inclusive and is preferably about 130 Å thick. A contact layer32 of p-type GaN is provided on the layer 30 and is preferably about1800 Å thick. Ohmic contacts 24 and 25 are provided on the p-GaN contactlayer 32 and the substrate 10, respectively.

FIG. 2 illustrates further embodiments of the present inventionincorporating a multi-quantum well active region. The embodiments of thepresent invention illustrated in FIG. 2 include a layered semiconductorstructure 100 comprising gallium nitride-based semiconductor layersgrown on a substrate 10. As described above, the substrate 10 may beSiC, sapphire or bulk gallium nitride. As is illustrated in FIG. 2, LEDsaccording to particular embodiments of the present invention may includea conductive buffer layer 11, a first silicon-doped GaN layer 12, asecond silicon doped GaN layer 14, a superlattice structure 16comprising alternating layers of silicon-doped GaN and/or InGaN, anactive region 125 comprising a multi-quantum well structure, an undopedGaN or AlGaN layer 22, an AlGaN layer 30 doped with a p-type impurity,and a GaN contact layer 32, also doped with a p-type impurity. The LEDsmay further include an n-type ohmic contact 23 on the substrate 10 and ap-type ohmic contact 24 on the contact layer 32. In embodiments of thepresent invention where the substrate 10 is sapphire, the n-type ohmiccontact 23 would be provided on n-type GaN layer 12 and/or n-type GaNlayer 14.

As described above with reference to FIG. 1, buffer layer 11 ispreferably n-type AlGaN. Similarly, GaN layer 12 is preferably betweenabout 500 and 4000 nm thick inclusive and is most preferably about 1500nm thick. GaN layer 12 may be doped with silicon at a level of about5×10¹⁷ to 5×10¹⁸ cm⁻³. GaN layer 14 is preferably between about 10 and500 Å thick inclusive, and is most preferably about 80 Å thick. GaNlayer 14 may be doped with silicon at a level of less than about 5×10¹⁹cm⁻³. The superlattice structure 16 may also be provided as describedabove with reference to FIG. 1.

The active region 125 comprises a multi-quantum well structure thatincludes multiple InGaN quantum well layers 120 separated by barrierlayers 118. The barrier layers 118 comprise In_(X)Ga_(1−X)N where 0≦X<1.Preferably the indium composition of the barrier layers 118 is less thanthat of the quantum well layers 120, so that the barrier layers 118 havea higher bandgap than quantum well layers 120. The barrier layers 118and quantum well layers 120 may be undoped (i.e. not intentionally dopedwith an impurity atom such as silicon or magnesium). However, it may bedesirable to dope the barrier layers 118 with Si at a level of less than5×10¹⁹ cm⁻³, particularly if ultraviolet emission is desired.

In further embodiments of the present invention, the barrier layers 118comprise Al_(X)In_(Y)Ga_(1−X−Y)N where 0<X<1, 0≦Y<1 and X+Y≦1. Byincluding aluminum in the crystal of the barrier layers 118, the barrierlayers 118 may be lattice-matched to the quantum well layers 120,thereby providing improved crystalline quality in the quantum welllayers 120, which increases the luminescent efficiency of the device.

Referring to FIG. 3, embodiments of the present invention that provide amulti-quantum well structure of a gallium nitride based device areillustrated. The multi-quantum well structure illustrated in FIG. 3 mayprovide the active region of the LEDs illustrated in FIG. 1 and/or FIG.2. As seen in FIG. 3, an active region 225 comprises a periodicallyrepeating structure 221 comprising a well support layer 218 a havinghigh crystal quality, a quantum well layer 220 and a cap layer 218 bthat serves as a protective cap layer for the quantum well layer 220.When the structure 221 is grown, the cap layer 218 b and the wellsupport layer 218 a together form the barrier layer between adjacentquantum wells 220. Preferably, the high quality well support layer 218 ais grown at a higher temperature than that used to grow the InGaNquantum well layer 220. In some embodiments of the present invention,the well support layer 218 a is grown at a slower growth rate than thecap layer 218 b. In other embodiments, lower growth rates may be usedduring the lower temperature growth process and higher growth ratesutilized during the higher temperature growth process. For example, inorder to achieve a high quality surface for growing the InGaN quantumwell layer 220, the well support layer 218 a may be grown at a growthtemperature of between about 700 and 900° C. Then, the temperature ofthe growth chamber is lowered by from about 0 to about 200° C. to permitgrowth of the high-quality InGaN quantum well layer 220. Then, while thetemperature is kept at the lower InGaN growth temperature, the cap layer218 b is grown. In that manner, a multi-quantum well region comprisinghigh quality InGaN layers may be fabricated.

For example, in order to achieve a high quality surface for growing theInGaN quantum well, the well support layer is grown at a growthtemperature of between 750 and 900° C. Then, the temperature of thegrowth chamber is lowered by about 50° C. to permit growth of ahigh-quality InGaN quantum well layer. Then, while the temperature iskept at the lower InGaN growth temperature, the cap layer is grown.

The active regions 125 and 225 of FIGS. 2 and 3 are preferably grown ina nitrogen atmosphere, which may provide increased InGaN crystalquality. The barrier layers 118, the well support layers 218 a and/orthe cap layers 218 b may be between about 50–400 Å thick inclusive. Thecombined thickness of corresponding ones of the well support layers 218a and the cap layers 218 b may be from about 50–400 Å thick inclusive.Preferably, the barrier layers 118 the well support layers 218 a and/orthe cap layers 218 b are greater than about 90 Å thick and mostpreferably are about 225 Å thick. Also, it is preferred that the wellsupport layers 218 a be thicker than the cap layers 218 b. Thus, the caplayers 218 b are preferably as thin as possible while still reducing thedesorption of Indium from or the degradation of the quantum well layers220. The quantum well layers 120 and 220 may be between about 10–50 Åthick inclusive. Preferably, the quantum well layers 120 and 220 aregreater than 20 Å thick and most preferably are about 25 Å thick. Thethickness and percentage of indium in the quantum well layers 120 and220 may be varied to produce light having a desired wavelength.Typically, the percentage of indium in quantum well layers 120 and 220is about 25–30%, however, depending on the desired wavelength, thepercentage of indium has been varied from about 5% to about 50% .

In preferred embodiments of the present invention, the bandgap of thesuperlattice structure 16 exceeds the bandgap of the quantum well layers120. This may be achieved by by adjusting the average percentage ofindium in the superlattice 16. The thickness (or period) of thesuperlattice layers and the average Indium percentage of the layersshould be chosen such that the bandgap of the superlattice structure 16is greater than the bandgap of the quantum wells 120. By keeping thebandgap of the superlattice 16 higher than the bandgap of the quantumwells 120, unwanted absorption in the device may be minimized andluminescent emission may be maximized. The bandgap of the superlatticestructure 16 may be from about 2.95 eV to about 3.35 eV. In a preferredembodiment, the bandgap of the superlattice structure 16 is about 3.15eV.

In additional embodiments of the present invention, the LED structureillustrated in FIG. 2 includes a spacer layer 17 disposed between thesuperlattice 16 and the active region 125. The spacer layer 17preferably comprises undoped GaN. The presence of the optional spacerlayer 17 between the doped superlattice 16 and active region 125 maydeter silicon impurities from becoming incorporated into the activeregion 125. This, in turn, may improve the material quality of theactive region 125 that provides more consistent device performance andbetter uniformity. Similarly, a spacer layer may also be provided in theLED structure illustrated in FIG. 1 between the superlattice 16 and theactive region 18.

Returning to FIG. 2, the layer 22 may be provided on the active region125 and is preferably undoped GaN or AlGaN between about 0 and 120 Åthick inclusive. The layer 22 is preferably about 35 Å thick. If thelayer 22 comprises AlGaN, the aluminum percentage in such layer ispreferably about 10–30% and most preferably about 24%. The level ofaluminum in the layer 22 may also be graded in a stepwise orcontinuously decreasing fashion. The layer 22 may be grown at a highertemperature than the growth temperatures in the active region 125 inorder to improve the crystal quality of the layer 22. Additional layersof undoped GaN or AlGaN may be included in the vicinity of layer 22. Forexample, the LED illustrated in FIG. 2 may include an additional layerof undoped AlGaN about 6–9 Å thick between the active regions 125 andthe layer 22.

An AlGaN layer 30 doped with a p-type impurity such as magnesium isprovided on layer 22. The AlGaN layer 30 may be between about 0 and 300Å thick inclusive and is preferably about 130 Å thick. A contact layer32 of p-type GaN is provided on the layer 30 and is preferably about1800 Å thick. Ohmic contacts 24 and 25 are provided on the p-GaN contactlayer 32 and the substrate 10, respectively. Ohmic contacts 24 and 25are provided on the p-GaN contact layer 32 and the substrate 10,respectively.

While embodiments of the present invention have been described withmultiple quantum wells, the benefits from the teachings of the presentinvention may also be achieved in single quantum well structures. Thus,for example, a light emitting diode may be provided with a singleoccurrence of the structure 221 of FIG. 3 as the active region of thedevice. Thus, while different numbers of quantum wells may be utilizedaccording to embodiments of the present invention, the number of quantumwells will typically range from 1 to 10 quantum wells.

While embodiments of the present invention have been described withreference to gallium nitride based devices, the teachings and benefitsof the present invention may also be provided in other Group IIInitrides. Thus, embodiments of the present invention provide Group IIInitride based superlattice structures, quantum well structures and/orGroup III nitride based light emitting diodes having superlatticesand/or quantum wells.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

1. A Group III nitride based light emitting diode, comprising: a GroupIII nitride based superlattice comprising a gallium nitride basedsuperlattice having at least two periods of alternating layers ofIn_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N, where 0≦X<1 and 0≦Y<1 and X is notequal to Y; and a Group III nitride based active region on thesuperlattice comprising at least one quantum well structure comprising:a first Group III nitride based barrier layer comprising a first wellsupport layer comprising a Group III nitride; a first Group III nitridebased quantum well layer on the first barrier layer; and a second GroupIII nitride based barrier layer on the first Group III nitride basedquantum well layer comprising a first cap layer comprising a Group IIInitride on the first quantum well layer; a second well support layercomprising a Group III nitride on the cap layer; a second quantum welllayer comprising a Group III nitride on the second well support layer;and a second cap layer comprising a Group III nitride on the secondquantum well layer; and wherein the cap layers have a lower crystalquality than the well support layers.
 2. The light emitting diode ofclaim 1, wherein the at least one quantum well structure comprises fromabout 2 to about 10 repetitions of the at least one quantum wellstructure.
 3. The light emitting diode according to claim 1, wherein thewell support layer comprises a gallium nitride based layer, the quantumwell layer comprises an indium gallium nitride layer and the barrierlayer comprises a gallium nitride based layer.
 4. The light emittingdiode according to claim 1, wherein the well support layer and the caplayer comprises a layer of In_(X)Ga_(1−X)N where 0≦X<1.
 5. The lightemitting diode according to claim 4, wherein an indium composition ofthe well support layer and the cap layer is less than an indiumcomposition of the quantum well layer.
 6. The light emitting diodeaccording to claim 1, wherein the well support layer and the cap layercomprises a layer of Al_(X)In_(Y)Ga_(1−X−Y)N where 0<X<1, 0≦Y<1 andX+Y≦1.
 7. The light emitting diode according to claim 6, whereinX≦Y+0.05.
 8. The light emitting diode according to claim 1, wherein thewell support layers and the cap layers are undoped.
 9. The lightemitting diode according to claim 1, wherein the well support layers andthe cap layers have a doping level of less than about 5×10¹⁹ cm⁻³. 10.The light emitting diode according to claim 1, wherein the cap layersand the well support layers have a higher bandgap than the quantum welllayers.
 11. The light emitting diode according to claim 1, wherein acombined thickness of the second well support layer and the first caplayer is from about 50 to about 400 Å.
 12. The light emitting diodeaccording to claim 1, wherein a thickness of the well support layer isgreater than a thickness of the cap layer.
 13. The light emitting diodeaccording to claim 1, wherein the quantum well layers have a thicknessof from about 10 to about 50 Å.
 14. The light emitting diode accordingto claim 1, wherein the quantum well layers have a thickness of about 25Å.
 15. The light emitting diode according to claim 1, wherein apercentage of indium in the quantum well layers is from about 5% toabout 50%.
 16. The light emitting diode according to claim 1, furthercomprising a Group III nitride based spacer layer between the first wellsupport layer and the superlattice.
 17. The light emitting diodeaccording to claim 16, wherein the spacer layer comprises undoped GaN.18. The light emitting diode according to claim 1, wherein a bandgap ofthe at least one quantum well is less than a bandgap of thesuperlattice.
 19. The light emitting diode according to claim 1, whereinthe gallium nitride based superlattice comprises from about 5 to about50 periods.
 20. The light emitting diode according to claim 1, whereinlayers of In_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N of the alternating layersof In_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N have a combined thickness of lessthan about 70 Å.
 21. The light emitting diode according to claim 1,wherein X=0.
 22. The light emitting diode according to claim 21, whereinInGaN layers of the alternating layers of In_(X)Ga_(1−X)N andIn_(Y)Ga_(1−Y)N have a thickness of from about 5 to about 40 Å and GaNlayers of the alternating layers of In_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)Nhave a thickness of from about 5 to about 100 Å.
 23. The light emittingdiode according to claim 21, wherein InGaN layers of the alternatinglayers of In_(X)Ga_(1−X)N and In_(Y)Ga_(1−Y)N have a thickness of about15 Å and GaN layers of the alternating layers of ln_(X)Ga_(1−X)N andIn_(Y)Ga_(1−Y)N have a thickness of from about 30 Å.
 24. The lightemitting diode according to claim 1, wherein the gallium nitride basedsuperlattice is doped with an n-type impurity to a level of from about1×10¹⁷ cm⁻³ to about 5×10 cm⁻³.
 25. The light emitting diode accordingto claim 24, wherein the doping level of the gallium nitride basedsuperlattice is an actual doping level of layers of the alternatinglayers.
 26. The light emitting diode according to claim 24, wherein thedoping level is an avenge doping level of layers of the alternatinglayers.
 27. The light emitting diode according to claim 1, furthercomprising doped Group III nitride layers adjacent the superlattice andwherein the doped Group III nitride layers are doped with an n-typeimpurity to provide an average doping of the doped Group III nitridelayers and the superlattice of from about 1×10¹⁷ cm⁻³ to about5×10¹⁹cm⁻³.
 28. The light emitting diode according to claim 1, wherein abandgap of the superlattice is from about 2.95 eV to about 3.35 eV. 29.The light emitting diode according to claim 1, wherein a bandgap of thesuperlattice is about 3.15 eV.
 30. A Group III nitride basedsemiconductor device having an active region comprising at least twostacked quantum well structures, the quantum well structures eachcomprising: a well support layer comprising a Group III nitride; aquantum well layer comprising a Group III nitride on the well supportlayer; and a cap layer comprising a Group III nitride on the quantumwell layer; and wherein the cap layer has a lower crystal quality thanthe well support layer.
 31. The Group III nitride based semiconductordevice according to claim 30, wherein the well support layer comprises agallium nitride based layer, the quantum well layer comprises an indiumgallium nitride layer and the barrier layer comprises a gallium nitridebased layer.
 32. The Group III nitride based semiconductor deviceaccording to claim 31, wherein the well support layer and the cap layercomprises a layer of In_(X)Ga_(1−X)N where 0≦X<1.
 33. The Group IIInitride based semiconductor device according to claim 32, wherein anindium composition of the well support layer and the cap layer is lessan indium composition of the quantum well layer.
 34. The Group IIInitride based semiconductor device according to claim 31, wherein thewell support layer and the cap layer comprises a layer ofAl_(X)In_(Y)Ga_(1−X−) _(Y)N where 0<X<1, 0≦Y<1 and X+Y≦1.
 35. The GroupIII nitride based semiconductor device according to claim 34, whereinX≦Y+0.05.
 36. The Group III nitride based semiconductor device accordingto claim 31, wherein the well support layer and the cap layer areundoped.
 37. The Group III nitride based semiconductor device accordingto claim 31, wherein the well support layer and the cap layer have adoping level of less than about 5×10¹⁹ cm⁻³.
 38. The Group III nitridebased semiconductor device according to claim 30, wherein the cap layerand the well support layer have a higher bandgap than the quantum welllayer.
 39. The Group III nitride based semiconductor device according toclaim 31, wherein a combined thickness of the well support layer and thecap layer is from about 50 to about 400 Å.
 40. The Group III nitridebased semiconductor device according to claim 31, wherein a combinedthickness of the well support layer and the cap layer is greater thanabout 90 Å.
 41. The Group III nitride based semiconductor deviceaccording to claim 31, wherein a combined thickness of the well supportlayer and the cap layer is about 225 Å.
 42. The Group III nitride basedsemiconductor device according to claim 31, wherein a thickness of thewell support layer is greater than a thickness of the cap layer.
 43. TheGroup III nitride based semiconductor device according to claim 31,wherein the quantum well layer has a thickness of from about 10 to about50 Å.
 44. The Group III nitride based semiconductor device according toclaim 31, wherein the quantum well layer has a thickness of about 25 Å.45. The Group III nitride based semiconductor device according to claim31, wherein a percentage of indium in the quantum well layer is fromabout 5% to about 50%.
 46. The Group III nitride based semiconductordevice according to claim 31, further comprising a superlattice andwherein the well support layer is on the superlattice.
 47. The Group IIInitride based semiconductor device according to claim 46, wherein abandgap of the superlattice is from about 2.95 to about 3.35 eV.
 48. TheGroup III nitride based semiconductor device according to claim 46,wherein a bandgap of the superlattice is about 3.15 eV.
 49. The GroupIII nitride based semiconductor device according to claim 46, furthercomprising a Group III nitride based spacer layer between the wellsupport layer and the superlattice.
 50. The Group III nitride basedsemiconductor device according to claim 49, wherein the spacer layercomprises undoped GaN.
 51. The Group III nitride based semiconductordevice according to claim 46, wherein a bandgap of the at least onequantum well is less than a bandgap of the superlattice.
 52. The GroupIII nitride based semiconductor device according to claim 30 having fromabout 2 to about 10 repetitions of the quantum well structure.